The present invention relates to a special effect apparatus for a video signal which is capable of producing three dimensionally effected video signals, and more particularly to a video composing system which creates a composed image by composing a plurality of three-dimensionally effected video signals delivered from the special effect apparatus.
As is well known, a special effect apparatus produces a three-dimensional effected image, such as image rotation, size expansion, size compression and perspective depth, by writing a digitized video signal into a memory and reading out it with a specially modified read address. An example of such special effect apparatus is disclosed in U.S. Pat. No. 4,831,445, entitled "Special Effect System for Video Signal" which issued May 16, 1989.
In a further variation involving creating a new effect image by composing a plurality of, for example two, effected video signals produced by such a special effect apparatus, video images to be composed overlap each other on a TV screen. In such a case, according to a prior art technique, the order of precedence of the two overlapped images is previously determined and, thus, the video image having priority is displayed in the overlapping area. It is assumed that video images A and B, illustrated in FIGS. 1 (a) and (b), respectively, each having a depth representation (in the Z-direction) by the three-dimensional effect, are composed with priority of the video image A. Though a video image having less depth in the Z-direction should be displayed in the overlapping area notwithstanding the predetermined priority order, the prior art only provides such a composed image as shown in FIG. 2 which does not consider the difference in depth of the overlapping portions. Combining the two video images A and B and taking account of their respective positions in the Z-direction (depth) should give a composed video image shown in FIG. 3, in which a part a of the video image A, having a greater depth than the video image B is hidden behind the video image B. Accordingly, the prior art which selects the video image merely by priority between video image signals to be composed is unable to create an accurate and naturally appearing composed effect, such as is illustrated in FIG. 3.